As mobile devices have been increasingly developed and the demand for such mobile devices has increased, the demand for secondary batteries has sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and discharge voltage, into which much research has been carried out and which is now commercialized and widely used.
A secondary battery has attracted considerable attention as an energy source for power-driven devices, such as an electric bicycle (E-bike), an electric vehicle (EV), and a hybrid electric vehicle (HEV), as well as an energy source for mobile wireless electronic devices, such as a mobile phone, a digital camera, a personal digital assistant (PDA), and a laptop computer.
A small-sized battery pack, in which a battery cell is mounted, is used for small-sized devices, such as a mobile phone and a digital camera. On the other hand, a middle or large-sized battery pack, in which a battery pack including two or more battery cells (hereinafter, also referred to as a “multi-cell”) connected to each other in parallel and/or in series is mounted, is used for middle or large-sized devices, such as a laptop computer and an electric vehicle.
A lithium secondary battery exhibits excellent electrical properties as described above; however, the lithium secondary battery has low safety. For example, when abnormal operations, such as overcharge, overdischarge, exposure to high temperature, and electrical cut-off, of the lithium secondary battery occur, decomposition of active materials and an electrolyte, which are components of the battery, is caused with the result that heat and gas are generated and the high-temperature and high-pressure conditions caused by generation of the heat and the gas accelerate the above-mentioned decomposition. Eventually, a fire or explosion of the lithium secondary battery may occur.
For this reason, the lithium secondary battery is provided with a safety system, such as a protection circuit to interrupt electric current when the battery is overcharged or overdischarged or when overcurrent flows in the battery, a positive temperature coefficient (PTC) element whose resistance greatly increases so as to interrupt electric current when the temperature of the battery increases, and a safety vent to interrupt electric current or to exhaust gas when pressure increases due to generation of the gas. In case of a small-sized cylindrical secondary battery, for example, the PTC element and the safety vent are usually disposed at the top of an electrode assembly (a generating element) having a cathode/separator/anode structure, which is mounted in a cylindrical container. In case of a small-sized prismatic or pouch-shaped secondary battery, on the other hand, the protection circuit module and the PTC element are usually mounted at the upper end of a prismatic container or a pouch-shaped case, in which the generating element is mounted in a sealed state.
The safety-related problem of the lithium secondary battery is even more serious for a middle or large-sized battery pack having a multi-cell structure. Since a plurality of battery cells is used in the multi-cell battery pack, abnormal operation of some of the battery cells may cause abnormal operation of the other battery cells with the result that a fire or explosion of the battery pack may occur, which may lead to a large-scale accident. For this reason, the middle or large-sized battery pack is provided with a safety system, such as a battery management system (BMS), to protect the battery cells from overdischarge, overcharge, and overcurrent.
Meanwhile, as the lithium secondary battery is continuously used, i.e. as the lithium secondary battery is repeatedly charged and discharged, the generating element and electrical connection members are gradually degraded. For example, degradation of the generating element leads to decomposition of electrode materials and the electrolyte, by which gas is generated. As a result, the battery cell (the container or the pouch-shaped case) gradually swells. In a normal state of the lithium secondary battery, an active controller, such as the BMS, detects overdischarge, overcharge, or overcurrent of the battery pack. In a case in which the detected overdischarge, overcharge, or overcurrent of the battery pack is excessive, the active controller interrupts electrical connection in the battery pack to lower the risk of the battery pack.
In connection with this case, FIG. 1 is a typical view showing circuitry of a conventional battery pack. Referring to FIG. 1, a conventional battery pack 50 includes a battery module 100 constituted by a plurality of battery cells, a BMS 60 to detect information regarding an operation state of the battery module 100 and to control the battery module 100 based on the detected information, and a power connection and disconnection part (relay) 70 to perform connection and disconnection between the battery module 100 and an external input and output circuit (inverter) 80 according to an operation command of the BMS 60.
In a case in which the battery module 100 normally operates, the BMS 60 keeps the power connection and disconnection part 70 in an ON state. In a case in which abnormality of the battery module 100 is sensed, the BMS 60 switches the state of the power connection and disconnection part 70 to an OFF state to interrupt charge and discharge of the battery module 100. On the other hand, in a case in which the BMS 60 abnormally operates or does not operate at all, the BMS 60 does not perform any control. Consequently, the power connection and disconnection part 70 is kept in the ON state. As a result, charge and discharge of the battery module 100 are continuously performed even in such an abnormal state
In a case in which the active controller as described above is used, however, it is necessary to supply external electric current to the BMS. If no electric current is supplied to the BMS, therefore, the BMS may not protect the battery pack. That is, the active controller checks a charge state of the battery and controls the battery using an electric signal. However, it is necessary to supply power to the active controller. Consequently, the active controller cannot be a fundamental solution when power is not normally supplied to the active controller.
In addition, gas may leak from the lithium secondary battery or a fire or explosion of the lithium secondary battery may occur when the lithium secondary battery is overcharged. As the lithium secondary battery is used as a high-voltage, high-capacity battery pack for vehicles, safety of the lithium secondary battery is important in preventing injury of people and damage to vehicles.
For this reason, it is necessary to provide a protection device for the battery pack to prevent leakage of gas from the overcharged lithium secondary battery or occurrence of a fire or explosion of the lithium secondary battery.
As an example of the protection device, an electrode terminal connection portion between the battery cells may be broken by swelling force of the battery cells to cut-off an electric connection circuit of the battery pack.
In the above protection device, however, it is necessary to lower strength of the electrode terminals or to reduce the thickness of the electrode terminals such that the electrode terminal connection portion between the battery cells can be easily broken. The electrode terminals manufactured as described above have low resistance to external force, such as vibration or impact.
In addition, large force equivalent to tensile strength of the electrode terminals is needed for the protection device to break the electrode terminal connection portion. To this end, it is necessary for the battery cells to be considerably overcharged such that the battery cells excessively swell. As a result, gas may easily leak from the battery or a fire or explosion of the battery may easily occur.
Therefore, there is a high necessity for technology that is capable of fundamentally securing safety of the battery pack while solving the above problems.